In general, the present invention relates to the field of tin and tin-alloy plating. In particular, the present invention relates to the reduction of whisker formation in a tin or tin-alloy film.
Tin layers are typically used in the electronics industry to provide good solderability of components. For example, tin layers may be deposited on a copper lead frame to provide a solderable finish. Unfortunately, tin layers, particularly electrodeposited tin layers, are subject to spontaneous whisker formation. “Whiskers” are hair-like single crystals that grow from the surface of the tin layer. Tin whiskers may range in diameter from a few nanometers to several microns (6 nanometers to 6 microns being common) and can reach lengths of several millimeters. Such whiskers can create shorts and introduce noise into electronic circuitry, thus creating a reliability problem in electronic devices.
Conventionally, the use of tin-lead alloys has been provided as a solution to the tin whisker problem. Generally, it is agreed that such alloys are significantly less prone to whisker formation than tin itself. However, the current worldwide activities to ban the use of lead have caused the tin whisker problem to resurface as lead-free tin deposits are increasingly used.
It is believed that tin whiskers form as a result of stresses in the tin or tin-alloy layer, although bulk diffusion of tin is also believed to be involved. However, the precise growth mechanism of tin whiskers is not fully understood. A number of stress-causing factors have been postulated, including lattice stresses due to the presence of impurity atoms in the tin layer, residual stresses due to tin plating conditions, stresses due to mechanical loading or working of the tin layer, stresses due to interaction with adjacent layers, such as intermetallic compound formation, differences in thermal expansion and the like, among others. See, for example, Ewell et al., Tin Whiskers and Passive Components: A Review of the Concerns, 18th Capacitor and Resistor Technology Symposium, pp 222-228, March, 1998.
Lindborg, in A Model for the Spontaneous Growth of Zinc, Cadmium and Tin Whiskers, Metallurgica, vol. 24, pp 181-186, 1976, postulates a two-stage model for whisker formation. The first stage involves diffusion of vacancies away from the place of whisker growth, leading to a counter-flow of tin to the whisker. This is followed by dislocation motion, possibly by grain boundary sliding, as the second stage. Thus, the whisker growth process can be understood as a combination of lattice rearrangements in the tin lattice and a second step where a sub-unit of the tin is not rearranging anymore but breaks out of the bulk deposit representing thereby the beginning of whisker growth. Lee et al. observed that whiskers grow in characteristic angles from the deposit, Lee et al., Spontaneous Growth mechanism of Tin Whiskers, Acta Mater., vol. 46, no. 10, pp 3701-3714, 1998. Lee et al. note that the slip systems in tin are {100} {001} and {100} {010}. By the combination of these two features, Lee et al. were able to show that whiskers start to grow from grains that have a different crystal growth orientation than the major orientation of the tin film. They explained whisker growth by different elongations of such grains resulting in cracks in the surface oxide layer. In such cases, the tin surface oxide film can be sheared along the boundaries of the grains. Whiskers then grow from the grain whose surface oxide is sheared, in order to release the compressive stress in the tin film. However, this contradicts the observation that tin deposits grow whiskers even when they are stored under high-vacuum conditions where the formation of an oxide layer is prevented. Both of the above-described articles fail to provide a method for overcoming the problem of tin whisker formation.
An approach to reducing tin whisker formation has been to use relatively thick tin layers, for example, layers of about 10 microns thickness. However, such thick layers are not always practical or they may be too thick for certain applications, such as current microelectronic applications.
U.S. Pat. No. 5,750,017 (Zhang) discloses a method of electrodepositing tin or tin alloy onto a metal substrate using pulse plating conditions. Such plating method provides a tin layer typically having a thickness of 3 to 6 microns wherein the grain size of the tin is from 2 to 8 microns. Although such grain size is postulated by Zhang to reduce tin whisker formation, such tin deposits still suffer from tin whisker growth.
There is thus a need for methods of providing tin and tin alloy layers, particularly thin layers, having reduced tin whisker formation and with reduced deposit cracks.